1 1/8" (28.6mm) ZS headset Replacement part for many Trek road bikesSteerer outer diameter 28.6 mm (1-1/8")Crown race inner diameter 30 mmCup Insertion OD Semi-integratedHeadtube ID 41 mmStack height 9 mmBearing type SealedHeadset type upper Unthreaded fully integratedHeadset type lower Semi-integrated